Send username for LTI 1.3 tools in membership service

The membership service can be used for provisioning users in the tool, however currently Moodle only seems to send the username of a user for non LTI 1.3 tools, which makes provisioning those users quite hard. It would be nice if either the username could be send as is done in the member_to_jsonld method, or if the full custom substitution parameters could be provided (this seems like what MDL-55898 was trying to achieve but it seems stuck).